Assay techniques generally relate to determination of the composition of a sample (e.g., whether or not a component of interest is in the sample, concentration of the component, etc.). Assays are commonly employed in biological and chemical applications. For example, molecular recognition often provides specificity and selectivity for important biological processes. Such molecular recognition is exploited in assay techniques such as those based on DNA hybridization microarrays and protein arrays.
Present day assay techniques are often implemented using an array of sensors, each sensor corresponding to a separate assay sample. The use of such a sensor array allows for many different assays to be carried out in parallel, and the resulting improvement in throughput is typically of great practical significance.
Accordingly, various technological approaches for providing suitable sensor arrays have been developed in the art. One such approach is magnetic tagging, as considered in U.S. Pat. No. 5,981,297 and U.S. 2004/0120185. Magnetic biosensors are under active development, and may soon rival established biological detection methods involving surface-bond fluorescent tags. For example, suppose it is desired to sense the presence of a particular DNA oligomer type X in a liquid sample. This can be accomplished via the following steps: 1) tag the oligomers in the sample with magnetic tags; 2) attach a probe oligomer to a sensor surface, where the probe oligomer is complementary to type X; 3) sense the presence or absence of a magnetic tag at the sensor. Since only oligomers of type X can hybridize to the probe at the sensor surface, the presence or absence of type X oligomers can be determined by sensing the tags with a magnetic sensor.
Since it is often desirable to perform assays on a large number of samples, methods for rapidly reading out results from a sensor array have been considered. For example, U.S. Pat. No. 3,941,982, U.S. Pat. No. 5,272,476, U.S. Pat. No. 6,535,822, U.S. Pat. No. 6,765,699 and U.S. 2004/0033627 consider sensor array readout in various contexts. Some of these references relate to time division multiplexing, where the outputs from several sensor elements are combined onto a single array output by time division multiplexing. For example, the common situation where sensor array elements are addressed sequentially by rows and columns is an instance of time division multiplexing.
However, certain problems relating to sensor arrays are not addressed by these methods. In particular, it is important to consider the time it takes a tagged part of the sample (i.e., a tagged component) to bind to the sensor surface. Typically, the sensor surface is sensitized with a complimentary oligomer probe. For example, consider a sensor element in proximity to a oligomer probe spot having a diameter of 200 μm. Such an oligomer probe spot is often deposited robotically and includes many identical copies of the oligomer. If the sensitized sensor element is comparable in size to the oligomer spot, then the tagged component will bond to the sensor surface relatively quickly. Conversely, if the sensitized sensor element is much smaller than the DNA spot, then the tagged component will bond to the sensor surface relatively slowly. Since the tagged component diffuses randomly above the oligomer probe spot, it will take longer to encounter a small sensor than a large sensor. However, a large sensor tends to be less sensitive than a small sensor. Therefore, conventional assay arrays typically require a design that balances a desire for high sensitivity (i.e., small sensors) with low binding delay (i.e. large sensors).
Modification of prior art sensor arrays to address this problem (e.g., by providing several sensor element pixels per oligomer spot in the preceding example) is not straightforward. For example, such provision of additional sensor element pixels can significantly and undesirably increase readout time. Accordingly, it would be an advance in the art to provide a sensor array for magnetically tagged samples providing rapid and sensitive detection of tagged components.